1. Field of Invention
The present disclosure relates to a method of measuring subsidence and compaction with a gravimeter. More specifically, the present disclosure relates to a method of measuring subsidence and compaction by measuring gravitational forces at locations in a wellbore over time.
2. Description of Prior Art
Hydrocarbons from subterranean strata are typically produced from formations in the strata by forming wells that intersect one or more of the formations. Pressure in the reservoir and the stress in the rock balance the overburden pressure and the reservoir stays in equilibrium and maintains its shape. Due to the production of fluids from the subterranean formation, such as water and/or hydrocarbon trapped in the formation, pressure in the formation is reduced. If strength of the rock is insufficient, the rock strain is increased in the vertical direction causing compaction within the reservoir.
Compaction is sometimes estimated by measuring the change in distance between two or more features or markers in the formation. These features are usually radioactive “bullets” shot through the casing into the formation. This distance between markers is measured by a tool with multiple detectors. Distances between adjacent features that are much larger or smaller than accuracy of the detectors requires moving the tool, which is usually deployed on wireline. The inherent elasticity of wireline reduces the accuracy with which the feature distance can be estimated.
Subsidence typically refers to a change in depth or elevation of the earth that is above the reservoir. Subsidence can be caused by fluid withdrawal from a reservoir, but there are other mechanisms. Above a reservoir, subsidence is generally not accompanied by compaction. However, within the reservoir, compaction and subsidence may occur at the same depth. Methods for measuring subsidence at the surface include tracking the elevation of a global positioning satellite (“GPS”) receiver; where the receiver can be mounted permanently at the surface or stationed periodically at the same location. For greater accuracy, a difference method using two GPS receivers can be used. One drawback of GPS is that it is not operational below the surface.